SWR Changes When I Increase My Transmit Power

Have you ever tuned up your rig running low power only to have it change when you increased the power?  (It is recommended that you reduce you rigs power to its lowest setting when you tune up your antenna.  This is so you will not cause accidental interference to ongoing QSOs or overheat your transmitter.)  This becomes more of a problem as you begin to work the HF bands, but it may also happen at VHF/UHF particularly if you run a hundred watts or more.

What is the cause of this problem?

The cause is usually the RF from your antenna is coupling into some surrounding conductor that is not part of the antenna.  This can be almost any piece of metal from a nearby wire to a metal building.  It may not be noticeable on all bands but the cause is still the same.  You may think the wire clothes line below your antenna is not a problem.  But if it is within 15% of some part of the wave length you’re running, it may become resonant and start to radiate thus effecting you antenna.  Remember this is how a yagi works.  Wire or long metal objects such are metal rain gutter running in parallel to the antenna are most often the cause for most hams.

Should you be concerned?

Probably yes.  1) It will impact the way your signal leaves you antenna thus resulting in your antenna “not working as advertised”.  2) It can cause all the problems associated with high SWR.  3) Depending on what it has coupled to, there is a potential for someone getting RF burns, could cause RF on the feed line, and/or RF interference with other electronic equipment.

What is one to do? 

The easiest solution is move the antenna.  Try to determine what the antenna is coupling to.  Metal objects are almost always the offender.  Hidden wiring in building walls/ceilings or metal siding/roof are some of the more common problems.  However, sometimes trees can cause this especially if they are beside the antenna.  Trees at the ends of dipoles aren’t usually a problem nor are metal masts/towers.  It is always good practice to use several feet of rope at the end of an antenna to isolate it from whatever you use to support it.  (Don’t forget to use an insulator.  Wet rope will conduct RF.) 

However, things are not as bad as they might seem though.  Generally moving one end of the antenna a few feet away from the offending conductor is sufficient to resolve the problem.

SWR: Coax Check

Even a new coaxial cable should be checked.  The coax and connectors may look flawless, check fine with an Ohm meter and still have a serious mismatch, i.e. high SWR. 

You will need a 50 Ohm dummy load, a SWR meter, a known good coax jumper, Ohm meter, and transmitter.

Ohm meter tests:

Check the cable from the center pin on one connector to the center pin of the connector on the other end.  These should be a short circuit, less than one ohm[1].  Next check the shield from the outside of one connector to the other.  This should also be a short circuit.  Now check from the center pin to the shield using either connector.  This should be an open circuit, no reading, on the highest Ohms scale[2].  Be sure that you are not measuring you fingers by coming in contact with the meter probe tips or any part of the connectors when you make this measurement.  Stay off the metal.  When these checks have been made and the connection are all correct you may proceed to the SWR check.

SWR meter tests:

Connect the dummy load to one end of the coax you want to test.  Connect the other end of the coax to the Antenna connection on the SWR meter.  Connect the jumper to the Transmitter connection on the SWR meter.  Connect the other end of the jumper to the transmitter’s output connector.

Turn the transmitter power to minimum and set the mode to CW, AM or FM.  SSB will not work.  Now transmit and read the SWR.  It should be no more than 1.1 to 1 at the frequency that you intend to use the coax.   

If the SWR is higher, carefully do a visual check of the connectors and run the Ohm meter test while wiggling the connectors.  This is especially true for a home build coaxial cable.  It is very easy to short coax when installing connectors of any type.  A cable may not be shorted or open but still have SWR problems.

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[1] - 75 feet of RB-8x measured 0.6 Ohms on both the shield and the center conductor.  When the leads are shorted, they measure 0.3 Ohms.  That means the coax is 0.3 Ohms.  This is acceptable

[2] - The meter I use has a 20 MOhm scale.  It has no reading on the same piece of coax.  This is fine.  You can check the meter by wetting a finger on each hand and measuring your resistance to assure that the meter is working.  This reading will vary widely and will even change depending on how tightly you hold the meter lead tips.

SWR – Antenna Adjustment

This procedure is for adjusting the SWR of an antenna using a cross-needle SWR meter.

Connect the antenna to one end of a coaxial line.  (It is best if it is the same line you intend to use during normal operations.)  Connect the other end of the coax to the connector on the SWR meter marked Antenna.  Now connect a short jumper (keep this coax jumper as short as practical) to the connector marked Transmitter on the SWR meter.   Connect the other end of the jumper to the transmitter output connector.  Set the transmitter to FM or CW.  Do not use the SSB mode.

Reduce the power out on the transmitter as low as it will go.  Now tune the transmitter to the low end of the band for which you are tuning the antenna.  Transmit and record the reading on the meter[1].  Next tune the transmitter to the middle of the band.  Transmit and record the reading.  Now tune the transmitter to the high end of the band.  Transmit and record the reading.

If the reading gets progressively higher from the first reading to the last, the antenna is too long and will need to be shortened.  If the reading gets progressively lower from the first reading to the last, the antenna is too short and will need to be lengthened.  If the second reading is lower than the first and last, the antenna is tuned in the band. 

If the SWR is still high where you intend to use the antenna the most, you may adjust the antenna to lower the SWR there, but it will change elsewhere in the band. 

If the SWR remains unacceptably high (greater than 2:1) at the lowest point, it means the antenna is resonant in the band of interest but the impedance of the antenna is not near 50 Ohms.  All the shortening or lengthening will not bring the SWR down any lower.  Other steps will need to be taken to adjust the antenna’s impedance.

Impedance adjustment will be discussed in future posts.

                                                                                 1.9:1 SWR

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[1]  - Read the SWR where the needles cross. 

SWR – General Information

First, let’s clear up a common misconception.  SWR does not tell you anything about how well your antenna “gets out”, i.e. radiates.  It is a measure of the quality of your antenna system¹ or its ability to transfer RF energy from your radio onto your antenna.   It is a very important measurement to make with any new antenna or coax.  It needs to be made periodically on any antenna system, especially with mobiles².

SWR tells you if your radio will be damaged by the antenna system.  There is another prevalent misconception one hears from time to time.  “The radios these days have protection circuits that cut back the power SO THE RADIO WON’T GET DAMAGED.”  This is half true.  The radios do have protection circuitry.  However, that circuitry will not protect the radio from long term abuse such as transmitting into a seriously mismatched antenna system.  Continuing to operate into a mismatch will “slow cook” your radio.  Just how long that will take is hard to judge.  If there is an emergency situation use the radio, but keep the transmissions as short as possible.  It is even more important that the radio not fail in that case.  The receiver will not be damaged by high SWR.  The receiver’s performance may drop slightly but your ears won’t notice.  However, if you do notice a performance change in your receiver, you can be pretty sure that there is something seriously wrong with the antenna system.  Do not transmit in this case.  There is something so seriously wrong that your signal will likely not to get out anyway.

If you are wondering if an SWR meter is a worthwhile investment let me put it this way.    Do you change the oil in your car regularly so the car will keep running?  Checking the SWR on your antenna system is the same kind of thing and you need the tools to do the job.  You need to invest in a SWR meter.

One thing about buying a SWR meter you need to know.  Be sure it covers the bands where you intend to use it.  Most of the moderately priced ones cover 160 meters through 6 or 2 meters and others cover 1.25 meters through something above 70 cm.  Be wary of ones that cover 160 or 80 meter through 70 cm.  These are either very expensive or are not very accurate especially on the 70 cm.   

Future posts will cover SWR measuring of coax and antennas.

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¹ - The term antenna system includes the antenna, the feed line (generally coax), and the connectors.

² - Most FM mobile radios indicate power out and not SWR on their displays.  Mobile antennas take a beating just from wind, bird strikes, trees, garage doors, etc.  I check the SWR around each federal holiday.  That is an easy reminder to make well space tests, ten times a year.